The principle objective of this research proposal is to evaluate the role of molecular damage and cellular lesions in radiation-induced cell death from the decay of the molecular damage producing agent, 125I. The nucleus is generally believed to contain the radiation-sensitive target(s) for cell death. 125I decay in the nucleus causes extensive DNA damage and is very cytotoxic but the mechanism(s) leading to cell death is unknown. In order to gain further insight into mechanism(s) of cell lethality, we propose to investigate the role of the location of lesion(s) produced by 125I decay in chromatin in relation to cellular radiosensitivity. Any modification of radiosensitivity by the extent of DNA damage (DNA single strand breaks, DNA double strand breaks and DNA-protein cross links) and repair (measured by alkaline and neutral filter elution) , repair patch distribution (measured by the buoyant density shift method) or chromatin compaction status (measured in cells with differing chromatin compaction due to their cell cycle position, chromatin decondensation produced by isoleucine deprivation, and DNA damage induction in isolated subcellular particles retaining different levels of chromatin compaction) relative to the ultrastructural location of the lesions in chromatin or proximity to potentially critical targets will be evaluated. The systematic evaluation of 125I decay-induced DNA damage and DNA damage repair in relation to the organization of chromatin and then correlation of this information with the cytotoxic endpoint of 125I decay in the cell nucleus may allow characterization of a critical lethal target(s) whose integrity is crucial for the successful reproductive functioning of the cell.